St. Louis Fire Department Capt. Garon Mosby said workers were using a forklift to try and switch out a propane tank when a leak from the tank ignited, creating a flash explosion. There wasn’t a fire, and the tank itself did not explode, but the flash was enough to injure the six workers, Mosby said.

Names of the victims have not been released, and the explosion is still being investigated.

Saia spokeswoman Jeannie Jump said in a statement that none of the injuries were life-threatening. The critically injured man, who is expected to survive, and three others were hospitalized with burns. All four hospitalized workers are forklift operators. Two other workers suffered minor injuries and declined treatment.

Jump said two of the hospitalized men were expected to be released on Friday.

The explosion was an isolated incident that never jeopardized the surrounding area, Jump said, and the company was “fully cooperating” with the investigation.

“Saia LTL Freight’s number one priority is the safety of our employees,” the statement said. “Our thoughts and prayers are with those injured in the accident as well as their families.”

Saia provides transportation and logistics services, operating 147 terminals in 34 states, with annual revenue of more than $1.1 billion. The company has 8,500 employees, including 154 who work at the terminal in St. Louis.